It has become desirable to employ non-impact electro-photographic printers for text and graphics. In such a printer, an electrostatic charge is formed on a photoreceptive surface of a moving drum or belt, and selected areas of the surface are discharged by exposure to light. A printing toner is applied to the drum and adheres to the areas having an electrostatic charge and does not adhere to the discharged areas. This method is referred to a "writing white". The toner is then transferred to a sheet of plain paper and is heat- and/or pressure-fused to the paper. By controlling the areas on the drum illuminated and the areas not illuminated characters, lines and other images may be produced on the paper. Other types of printers may use similar exposure, such as "writing black" which is essentially the reverse of the charging and discharging just mentioned, or for exposing photographic or other photosensitive materials.
One type of nonimpact printer employs an array of light emitting diodes (commonly referred to herein as LEDs) for exposing the photoreceptive surface. A row, or several closely spaced rows, of minute LEDs are positioned near an elongated lens array so that their images are arrayed across the surface to be illuminated. As the surface moves past the line of LEDs, they are selectively activated to either emit light or not, thereby exposing or not exposing, the photoreceptive surface in a pattern corresponding to the LEDs activated.
To form good images in a black and white LED printer, it is desirable that all of the light emitting diodes produce the same light output when activated. This assures a uniform quality image all the way across a paper. On the other hand, in a grey scale printer, the light output of each LED is controlled to obtained the appropriate density of image. Preferably an adjustable constant current source for each LED junction is used to obtain a desired light output as a function of current. LEDs are switched on or off as appropriate for the data being printed. If used in a gray scale printer, the total exposure on a pixel by an LED depends on the time interval when it is turned on and on the current through the LED. Data for driving the individual LEDs across a printhead is, therefore, often in the form of pulse width modulation, but may also be in the form of current modulation.
Light emitting diodes for print heads are formed on wafers of gallium arsenide or the like, suitably doped to conduct current and emit light. Long arrays of LEDs are formed on a wafer which is cut into separated dice, each having an array of LEDs. A row of such dice are assembled end-to-end to form a print head array.
The LEDs are driven by current sources on integrated circuit dice. Generally speaking each integrated circuit chip has adjustable means for setting a reference voltage for the current sources. On that chip each current source comprises a field effect transistor (FET) gated by the reference voltage. To provide a constant current, each current source also includes a data FET in series with the constant current FET which, in response to a data signal at its gate, interconnects the constant current FET to an individual LED on the chip connected to that integrated circuit. Such an arrangement is described and illustrated in U.S. Pat. application Ser. No. 07/300,004, filed Jan. 19, 1989, now U.S. Pat. No. 4,864,216 issued Sept. 5, 1989, and assigned to the assignee of this application.
Thus, in a typical LED printhead there is one constant current FET and one data FET for each LED. It turns out that the cost of providing this driver circuitry is a large fraction of the total cost of an LED printhead.
It has been observed that for many printing tasks, each constant current driver for an LED is on only a small fraction of the time. This invention, therefore, employs time multiplexing so that each current source can be shared among many LEDs, thereby resulting in very substantial cost reductions for an LED printhead.